1. Field of the Invention
The present invention relates to a color toner for forming an image and, more particularly, to a color toner suited for use in an image forming method employing a photofixing system. The color toner of the present invention can be used advantageously as a developing agent in various image forming apparatuses, employing various image forming systems such as electrography, electrophotography and ionography such as, for example, an electrophotographic copying machine, an electrophotographic facsimile, an electrophotographic printer and an electrostatic printing machine. The present invention also relates to a color image forming method and a color image forming apparatus which employ the color toner for forming an image.
2. Description of the Related Art
The image forming method employing electrophotography, which has been widely used in copying machines, printers and printing machines generally, begins by charging the surface of a photoconductive insulator such as a photosensitive drum uniformly with a positive or a negative electrostatic charge. After the uniform charging, the electrostatic charge on the insulating material is partially erased by irradiating the photoconductive insulator with image light, by various means, to thereby form an electrostatic latent image. For example, an electrostatic latent image corresponding to image information can be formed on the photoconductive insulator by erasing the surface charge from particular portions by irradiating it with a laser beam. Then a fine powder of developing agent, referred to as a toner, is caused to deposit on the latent image where the electrostatic charge remains on the photoconductive insulator, thereby to visualize the latent image. Lastly, in order to print the toner image obtained as described above, it is common to electrostatically transfer the image onto a recording medium such as recording paper.
On the other hand, according to image forming method employing ionography, ions are generated by an ion (charged particles) generating means employing a support drum having an electrostatic film as a dielectric member for supporting an electrostatic image and the electrostatic image is formed on the surface of the dielectric member by ions. The electrostatic image thus formed is visualized by developing with a toner. Finally, the resulting toner image is subjected to transfer and fixing steps in the same manner as in case of electrophotography described above to obtain a print.
In the image forming method described above, the fixing step is conducted by employing a common technique. For the fixation of the transferred toner image, such methods as a fixing method wherein toner is melted under pressure, by heating or by combination thereof and is then solidified or a fixing method wherein toner is melted by irradiating with light and is then solidified, have been employed, while much interest is being directed to a method referred to as a photofixing method (also referred to as a flash fixing method), which uses light, that is free from adverse effects of pressure and heating. In the photofixing method, since it is not necessary to apply pressure to the toner during fixation, the need to bring the toner into contact with a fixing roller or the like is eliminated, and such an advantage is provided that blurring of the image and dust do not occur and imaging resolution (reproducibility) suffers less degradation in the fixing step. Also because it is not necessary to heat the toner with a heat source, the idle time, before printing can be started, after turning on the power to preheat the heating medium (fixing roller or the like) to a predetermined temperature is eliminated, so that printing can be started upon turning on the power. Eliminating the need for a high temperature heat source has another advantage that the temperature in the apparatus does not rise too high. It also eliminates such a danger that the recording paper catches fire due to the heat generated by the heat source, even when the recording paper jams in the fixing device due to a system breakdown or other trouble. There is also an advantage that fixing performance is less affected by the type of material and thickness of the recording medium such as glued paper, preprint paper and sheets of paper having different thicknesses.
Describing in more detail, the process of fixing a toner onto a recording paper in the photofixing method is as follows.
In the transfer step, a toner image is transferred onto a recording paper from a photosensitive drum. At this point of time, the toner is deposited on the recording paper in the form of a powder image to form a image and is in the state where the image can be damaged when rubbed with fingers. Then, the toner powder image on the recording paper is irradiated with flashlight from a Xenon flashlight. As a result of absorption of a light energy of flashlight, the toner is heated, softened and deposited on the recording paper. When the temperature is reduced after irradiation with flashlight, the toner image is solidified, thereby to complete a fixed toner image.
It is important to prevent so-called poor fixation wherein folding and rubbing of the recording paper cause peeling of the fixed toner image from the recording paper, resulting in poor image quality. For this purpose, it is necessary to design that melting of the toner during fixation is accelerated by enhancing the light adsorption capability of the toner, and thus the toner sufficiently penetrates into the paper and is firmly fixed.
The Xenon flash lamp commonly used in the photofixing method has an emission distribution in a wide range from an ultraviolet region to an infrared region, and shows a particularly strong emission intensity in the near infrared region of wavelengths ranging from 800 to 1000 nm. Therefore, it is required to establish the technology of efficiently absorbing a light energy in this region in order to develop a toner having high fixability. Recently, the demand for color prints has particularly been increased. The colorant used in the color toner has an absorption in the visible region, but has low light absorption efficiency in the near infrared region. Therefore, it is required to put a color toner capable of affording good fixability in the photofixing system into practical use.
In a black toner, a black coloring material as the colorant also absorbs light in the near infrared region. Therefore, the black toner has already been put into practical use as an electrophotographic apparatus employing a photofixing system. However, a further improvement in absorption efficiency is required to cope with recent increase of the demand for energy saving.
In response to the demand, it has been proposed to improve the fixability by adding an infrared absorbing agent to the color toner, and many patent applications related to this technology have been publicly disclosed such as: Japanese Unexamined Patent Publication (Kokai) Nos. 60-63545, 60-63546, 60-57858, 60-57857, 58-102248, 58-102247, 60-131544, 60-133460, 61-132959, W099/13382, 2000-147824, 7-191492, 2000-155439, 6-348056, 10-39535, 2000-35689, 11-38666, 11-125930, 11-125928, 11-125929, and 11-65167. Technologies disclosed in these publications are attempts to make color rendering and photofixability compatible with each other by adding an infrared absorbing agent to the toner. However, all of the infrared absorbing agents proposed has the problem of inability to achieve satisfactory fixation. To satisfy the photofixability, a large amount of these infrared absorbing agents must be added in the color toner. It is not always possible to meet the demand because of these problems such as (i) adverse influence on the chargeability of the toner, (ii) adverse influence on the color hue of the toner and (iii) increase in cost of the toner.
Japanese Unexamined Patent Publication (Kokai) Nos. 11-24319, 9-328412, 11-24317, 7-90310 and 6-228604 have proposed that the surface of fine particles is provided with a multi-layer film by adding additives in the form of fine particles in place of the use of the infrared absorbing agents described above, thereby controlling an absorption wavelength of light. However, none of these publications suggest that an absorption wavelength of fine particles is controlled in the near infrared region according to an emission wavelength of a photofixing device, thereby improving the fixability of the photofixed toner.
An object of the present invention is to provide a color toner, for forming an image, which allows use of the photofixing system for fixing images and is superior in fixability of the color toner, and which can also achieve a long life due to charge stabilization and environmental stabilization.
Another object of the present invention is to provide a color toner, for forming an image, which allows use of the photofixing system for fixing images without forming voids and is superior in fixability of the color toner, and also which can maintain a stable charge and developing properties for a long period because of less variation in charge amount due to an infrared absorbing agent used, and causes less influence on color hue due to the infrared absorbing agent.
Further, another object of the present invention is to provide a color image forming method, which allows use of the photofixing system for fixing images and is superior in fixability of the color toner, and which can also achieve a long life due to charge stabilization and environmental stabilization.
Furthermore, another object of the present invention is to provide a color image forming apparatus, which allows use of the photofixing system for fixing images and is superior in fixability of the color toner, and which can also achieve a long life due to charge stabilization and environmental stabilization.
The objects described above and other objects of the present invention will become apparent from the following detailed description.
In one aspect thereof, the present invention resides in a color toner for forming an image, comprising at least a binder resin and a colorant, which is used in an image forming method employing a photofixing system, said color toner further comprising at least one additive selected from the group consisting of:
(1) light-absorbing fine composite particles comprising core particles and at least two coating layers having different refractive indexes laminated on the surface of the core particles;
(2) an azulene compound represented by the following general formula (I): 
wherein R1 to R12 may be the same or different and each represents a hydrogen atom, or a substituted, or unsubstituted, saturated or unsaturated aliphatic hydrocarbon group, X represents an anion, and n represents a positive integer;
(3) a cyanine compound represented by the following general formula (II): 
wherein R1 to R5 may be the same or different and each represents a hydrogen atom, a substituted or unsubstituted, saturated or unsaturated, aliphatic hydrocarbon group, an alicyclic hydrocarbon group, an aromatic hydrocarbon group, or an aromatic hydrocarbon group having an aliphatic hydrocarbon side chain, and X represents an anion;
(4) a quinoline compound represented by the following general formula (III): 
wherein RI represents a hydrogen atom, a halogen atom, or a monovalent substituent, RII and RIII may be the same or different and each represents a hydrogen atom, or a substituted or unsubstituted, saturated or unsaturated, aliphatic hydrocarbon group, M represents a transition metal atom, X represents an anion, and n represents a positive integer;
(5) a quinoline compound represented by the following general formula (IV): 
wherein L represents a bonding group, RII and RIII may be the same or different and each represents a hydrogen atom, or a substituted or unsubstituted, saturated or unsaturated, aliphatic hydrocarbon group, M represents a transition metal atom, X represents an anion, and n represents a positive integer;
(6) a polymethine compound represented by the following general formula (V): 
wherein R1 and R2 may be the same or different and each represents a hydrogen atom, a substituted or unsubstituted, saturated or unsaturated, aliphatic hydrocarbon group, an alicyclic hydrocarbon group, an aromatic hydrocarbon group, or an aromatic hydrocarbon group having an aliphatic hydrocarbon side chain, and X represents an anion;
(7) a benzene metal complex represented by the following general formula (VI): 
wherein R represents a monovalent substituent, A and B may be the same or different and each represents a sulfur atom, a selenium atom, or xe2x80x94NH, M represents a transition metal atom, and X represents a cation; and
(8) photothermal, i.e., light-heat conversion, ceramics.
In another aspect thereof, the present invention resides in a method of forming a color image on a recording medium by means of an electrophotographic system which comprises the steps of forming an electrostatic latent image by image exposure, visualizing the electrostatic latent image by development, transferring the visualized image onto the recording medium and fixing the transferred image, wherein
a developing agent containing the color toner of the present invention is used in the step of developing the electrostatic latent image, and
photofixation is conducted at a light emission energy density ranging from 1.0 to 6.0 J/cm2 in the step of fixing the transferred image after transferring the image visualized by using the developing agent onto the recording medium.
Further, in another aspect thereof, the present invention resides in an apparatus for forming a color image on a recording medium by means of an electrophotographic system, comprising an image exposing device for forming an electrostatic latent image, a developing device for visualizing the electrostatic latent image, an image transferring device for transferring the visualized image onto the recording medium, and an image fixing device for fixing the transferred image onto the recording medium, wherein
the developing device is loaded with a developing agent containing the color toner of the present invention, and
the image fixing device is provided with a photofixing device having a light emission energy density ranging from 1.0 to 6.0 J/cm2.